Nanostructured photodiode

ABSTRACT

The present invention provides a photodiode comprising a p-i-n or pn junction at least partly formed by first and second regions ( 2 ) made of semiconductor materials having opposite conductivity type, wherein the p-i-n or pn junction comprises a light absorption region ( 11 ) for generation of charge carriers from absorbed light. One section of the p-i-n or pn junction is comprises by one or more nanowires ( 7 ) that are spaced apart and arranged to collect charge carriers generated in the light absorption region ( 11 ). At least one low doped region ( 10 ) made of a low doped or intrinsic semiconductor material provided between the nanowires ( 7 ) and one of said first region ( 1 ) and said second region ( 2 ) enables custom made light absorption region and/or avalanche multiplication region of the active region ( 9 ).

TECHNICAL FIELD OF THE INVENTION

The present invention relates to photodiodes and avalanche photodiodesthat includes nanowires as part of the pn or pin junction.

BACKGROUND OF THE INVENTION

A photodiode (PD) is a type of photodetector that is capable ofgenerating charge carriers when exposed to light or otherelectromagnetic energy. An avalanche photodiode (APD) is another type ofphotodetector device that further to the charge carrier generationincludes a multiplication of the charge carriers, i.e. an internalcurrent gain effect, which enables a high sensitivity. Hereinafter PDsand APDs are commonly simply referred to as photodiodes when featurescommon for both types of photodetectors are discussed.

Semiconductor photodiodes fabricated using conventional planartechnology comprise a vertical pn junction, i.e. a p-type semiconductorlayer on an n-type semiconductor layer, or a vertical p-i-n junction,i.e. with one or more intermediate intrinsic or low doped semiconductorlayers in-between the p-type and n-type layers, in-between twoelectrical contacts. The avalanche photodiode performs amplification ofa photocurrent generated from absorbed light by applying a reverse biasvoltage to its p-i-n junction to cause an avalanche multiplication undera high electric field. It is well known that the performance ofsemiconductor photodiodes is limited by high dark current and noise.Typically different parts of the p-i-n or pn junction of APDs are formedof different materials in order to improve the performance of e.g. thelight absorption region and the avalanche multiplication region. Forexample Si provides low noise characteristics but limits the detectablewavelength. However, one significant contribution to the device leakagecurrent, i.e. the dark current, originates from a fairly high defectdensity in the device layers. This is predominant when device layersthat are non-compatible with respect to e.g. lattice strain arecombined, by epitaxial growth or by wafer bonding. Further, APDs sufferfrom edge breakdown due to the high electrical field. The edge breakdowncan partly be avoided by using so called guard rings. However thislimits the active area of the device.

The size of sensitive photodetector structures is currently limited dueto a disproportionate increase in dark current with an increase insurface area. In addition, the dark current tends to increase over timedue to degradation of the semiconductor material. Eventually, the devicefails due to short-circuiting. This effect is particularly prominent inhigh internal electric field devices such as APDs since high currentlevels will accelerate the degradation.

SUMMARY OF THE INVENTION

The prior art large area photodetectors have drawbacks with regards tohigh dark current, limited sensitivity, scalability and limitedreliability due to short circuiting. An object of the present inventionis to provide a photodiode that may at least partly overcome thedrawbacks of the prior art.

The photodiode according to the invention comprises a p-i-n or pnjunction at least partly formed by a first region made of asemiconductor material having a first conductivity type and a secondregion made of a semiconductor material having a second conductivitytype opposite to the first conductivity type arranged in-between twocontacts. The p-i-n or pn junction comprises a light absorption regionfor generation of charge carriers from absorbed light. One section ofthe p-i-n or pn junction comprises one or more nanowires that are spacedapart and arranged to collect the charge carriers from the lightabsorption region. Preferably the nanowires protrude from asemiconductor substrate or a surface layer arranged on the semiconductorsubstrate and the semiconductor substrate or the surface layer comprisessaid first region.

At least one low doped region made of a low doped or intrinsicsemiconductor material that is provided between the nanowires and one ofsaid first region and said second region enables an improved design andperformance of photodiodes and avalanche photodiodes.

In one aspect of the invention an axial photodiode design is provided,wherein said first region of the first conductivity type is a dopedregion in the semiconductor substrate or the surface layer and the lowdoped region is provided between said first region and the nanowires.

In another aspect of the invention a radial photodiode design isprovided, wherein the low doped region and optionally a doped region isarranged on each nanowire in a core-shell configuration.

The photodiode is not limited to these axial and radial designs. One ormore low doped regions can be provided on both sides of the nanowires,and a planar layer on one side of the nanowires can be combined with aradial layer on the other side of the nanowires.

In an avalanche photodiode according to one embodiment of the presentinvention a p-i-n junction is partly formed by a low doped region madeof a low doped or intrinsic semiconductor material arranged on a dopedregion of the first conductivity type in a semiconductor substrate or asurface layer arranged on the semiconductor substrate. The low dopedregion comprises an avalanche multiplication region and optionally alight absorption region. Another part of the p-i-n junction is formed byone or more nanowires protruding from the low doped region. Thenanowires may be doped to provide a second region of a secondconductivity type opposite to the first conductivity type or thenanowires may be connected to an adjacent doped layer that together withthe nanowires or by itself provides said second region.

In another embodiment of an avalanche photodiode according to thepresent invention a p-i-n junction is formed by a first low doped layermade of a low doped or intrinsic semiconductor material of secondconductivity type arranged on a first doped layer of the firstconductivity type in a semiconductor substrate or a surface layer,nanowires protruding through openings in a dielectric layer to a secondlow doped layer of a low doped or intrinsic semiconductor material ofthe second conductivity type opposite to the first conductivity type,and a second doped layer of the second conductivity type arranged on thesecond low doped layer. A light absorption region is provided by thesecond low doped layer and an avalanche multiplication region isprovided by the first low doped layer. Charge carriers generated fromlight absorbed in the second low doped layer are collected by thenanowires and transported to the first low doped layer due to a reversebias.

Many of the limitations in the commonly used planar technology can bederived from challenges in limited flexibility in material combinations,mainly due to lattice match restrictions. The possibility of directgrowth of e.g. III-V nanowires on silicon for fabrication of photodiodesaccording to the invention implies strong advantages as compared tocommonly used wafer-bonding techniques.

Thanks to the invention it is possible to provide photodiodes that haveimproved dark current properties. It is a further advantage of theinvention to provide photodiodes that have a substantially constant darkcurrent over time, thereby significantly reducing the risk for failuredue to short circuiting. Instead the nanowires of the photodiodesaccording to the invention may be designed to provide a circuit breakerbehavior.

The performance and reliability of photodiodes according to theinvention can be further improved by utilizing at least a portion of thenanowires as a current limiting structure based on space charge build-upin the nanowire portion. This space charge effect yields a surprisinglyhigh, and designable, series resistance which makes it possible toprevent excessive current leakage and device failure. Reverse resistanceof the photodiode may also be increased to enable photodiodes withhigher breakdown voltage and improved avalanche functionality.

Embodiments of the invention are defined in the dependent claims. Otherobjects, advantages and novel features of the invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described withreference to the accompanying drawings, wherein:

FIGS. 1 a-b schematically illustrate photodiodes wherein the activeregion is located in a common intrinsic layer in-between doped nanowiresand a doped substrate in accordance with the invention;

FIGS. 2 a-b schematically illustrate photodiodes comprising nanowiresprotruding from a doped substrate that forms one region of the p-i-njunction wherein the active region is located in intrinsic layersenclosing each nanowire, in FIG. 2 a the other doped region of the p-i-njunction is formed by a common coalesced semiconductor layer re-grownfrom nanowires, and in FIG. 2 b each nanowire is enclosed in a separatedoped semiconductor layer in accordance with the invention;

FIG. 3 schematically illustrate in (a) one embodiment of an axialphotodiode design and in (b) a radial photodiode design based on InGaAsnanowires grown on silicon in accordance with the invention;

FIG. 4 schematically illustrates a photodiode comprising two low dopedlayers separated by nanowires in accordance with the invention;

FIG. 5 schematically illustrates an InGaAs/Si SAM APD in accordance withthe invention;

FIG. 6 schematically illustrates (a) an electric field diagram and banddiagrams of the InGaAs/Si SAM APD of FIG. 5 (b) with no bias and (c)reverse biased; and

FIG. 7 schematically illustrates a junction profile under the nanowiresin accordance with the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In the present invention one section of a semiconductor photodiode ispartitioned by using a nanowire array comprising typically 1 to morethan 10exp8) nanowires spaced apart from each other. Basically, thenanowires of the nanowire array connect the p and n regions of thephotodiode to each other, whereby charge carriers, which are generatedfrom absorbed light in a light absorption region, are due to a biascollected by the nanowires and efficiently conducted away. This can beused for different photodetectors and is in the following particulardescribed for photodiodes (PD), avalanche photodiodes (APD), andseparate absorption-multiplication avalanche photodiode (SAM APD).

For the purpose of this application the term nanowire is to beinterpreted as a structure being essentially of nanometer dimensions inits width or diameter. Such structures are commonly also referred to asnanowhiskers, nanorods, etc. The basic process of nanowire formation onsubstrates by particle assisted growth or the so-called VLS(vapour-liquid-solid) mechanism described in U.S. Pat. No. 7,335,908, aswell as different types of Chemical Beam Epitaxy and Vapour PhaseEpitaxy methods, which are well known. However, the present invention islimited to neither such nanowires nor the VLS process. Other suitablemethods for growing nanowires are known in the art and is for exampleshown in international application No. WO 2007/104781. From this itfollows that nanowires may be grown without the use of a particle as acatalyst. Thus selectively grown nanowires and nanostructures, etchedstructures, other nanowires, and structures fabricated from nanowiresare also included.

While there are certain limitations in semiconductor devices fabricatedusing planar technology, such as lattice mismatch between successivelayers, nanowire technology provides greater flexibility in selection ofsemiconductor materials in successive layers and hence greaterpossibility to tailor the band structure. One example is growth of III-Vsemiconductor nanowires on Si substrates, which is utilized forembodiments of the invention that are described in the following.Nanowires potentially also have a lower defect density than planarlayers and by replacing at least portions of planar layers insemiconductor devices with nanowires, limitations with regards todefects can be diminished. Further, nanowires provide surfaces with lowdefect densities as templates for further epitaxial growth.

Referring to FIGS. 1-4, in detail a photodiode according to the presentinvention comprises a p-i-n or pn junction at least partly formed by afirst region 1 made of a semiconductor material having a firstconductivity type and a second region 2 made of a semiconductor materialhaving a second conductivity type opposite to the first conductivitytype, i.e. the first region is p-type and the second region is n-type orthe first region is n-type and the second region is p-type. The p-i-n orpn junction further comprises a light absorption region 11 forgeneration of charge carriers from absorbed light. One section of thep-i-n or pn junction comprises an array of nanowires 7 that are spacedapart and arranged to collect the charge carriers from the lightabsorption region 11. A gap between the nanowires may be filled with adielectric material, an intrinsic semiconductor material or a low dopedsemiconductor material. This material may be transparent.

Nanowires are typically grown on a semiconductor substrate or a surfacelayer arranged on the semiconductor substrate. The semiconductorsubstrate and/or the surface layer is preferably part of the photodiodeaccording to the invention. The nanowires 7 of the photodiode thenprotrudes from a semiconductor substrate 3 or a surface layer 4 arrangedon the semiconductor substrate 3 and the semiconductor substrate 3 orthe surface layer 4 comprises said first region 1.

As indicated above the photodiode may comprise a pn junction withsemiconductor materials of opposite conductivity type on each side of ajunction. Charge carriers generated due to absorption of incoming lightare separated by the junction and contribute to a photocurrent. Theabsorption takes place in a depletion region or in immediate vicinitythereof. The properties, such as the width, of the depletion region canbe changed by introducing a low doped region made of a low doped orintrinsic semiconductor material in-between the p-type and n-typeregion, preferably adjacent to the nanowires, thereby forming a p-i-njunction. Inherently the low doped region of the p-i-n junction enablesa wider depletion region than in the pn junction. The depletion region,and hence the light absorption region, may be fully within the low dopedregion or it may extend into any adjacent region such as the nanowiresand/or the p or n region. The major use of nanowires in this inventionis not to be light absorbers, although at least a part thereof can beused as such. Consequently, the light absorbing region may overlap withthe nanowires 7.

In one embodiment of the present invention at least a first low dopedregion 10 made of a low doped or intrinsic semiconductor material isprovided between the nanowires 7 and one of said first region 1 and saidsecond region 2 of opposite conductivity type in order to at leastpartly form the light absorption region 11 of a p-i-n junction. Lowdoped regions 10 can be provided on any or both sides of the nanowires7. For the purpose of this application a low doped region 10 arranged inthe semiconductor substrate or a surface layer arranged on thesemiconductor substrate is referred to as an axial design and a lowdoped region 10 arranged as a shell layer that at least partly encloseseach nanowire 7 is referred to as a radial design. However, theinvention is not limited to pure axial or radial designs. For example,different shell layers may be combined with low doped regions in thesemiconductor substrate. The low doped region contributes to theformation of a light absorption region, which is either substantiallylocated adjacent to, or in the semiconductor substrate, in a shell layersurrounding the nanowires or in the layer grown from the nanowires,although the light absorption region 9 may extend into the nanowires 7and/or said first and second regions 1,2.

Such a low doped region can also be used in the photodiode for theformation of an avalanche multiplication region in the p-i-n junction.The avalanche multiplication and the light absorption may be at leastpartly formed by the same or different low doped regions. Further theavalanche multiplication region may be at least partly formed by thenanowires.

FIG. 1 a schematically illustrates one embodiment of a photodiodeaccording to the present invention comprising a low doped region. Thephotodiode comprises an array of nanowires 7 protruding from a surfacelayer 4 arranged on a semiconductor substrate 3. As shown in FIG. 1 a,an end portion of each nanowire 7 may be connected to a common firstcontacting means 5. A p-i-n junction is formed by a first region 1 inthe semiconductor substrate 3 made of a p-doped semiconductor materialand a second region 2 in each nanowire 7 made of a n-doped semiconductormaterial with a low doped region 10 in the surface layer 4 made of a lowdoped or intrinsic material. An light absorption region 11 forgeneration of charge carriers from absorbed light is substantiallyprovided by the low doped region 10. The generated charge carriers arecollected by the nanowires 7 due to a reverse biased p-i-n junction. Thelight absorption region 11 may be formed by the low doped region 10itself or the light absorption region 11 may extend into the nanowires 7and/or the first region 1. FIG. 1 b schematically illustrates aphotodiode where the first and second regions 1,2 has opposite polarityas compared to the photodiode in FIG. 1 a, i.e. the semiconductorsubstrate 3 is n-type and the nanowires 7 are p-type. The nanowirestructures of FIGS. 1 a-b can be designed to be operated as photodiodesor avalanche photodiodes. The avalanche photodiode is operated withreverse bias and in addition to the light absorption region the lowdoped region 10 provides an avalanche multiplication region. A secondcontacting means (not shown) may be electrically connected to thebackside of the semiconductor substrate or elsewhere.

The use of nanowires 7 in the embodiment that is illustrated in FIG. 1enables a partitioning of the electrical conduction path without theloss of active volume. This partitioned photodiode demonstratesignificantly decreased dark current without loss of quantum efficiency,as compared to a corresponding planar device. While beingstraightforward to fabricate, the partitioned photodiode offers a simplesolution to the fundamental problem of high dark current of planardevices.

FIGS. 2 a-b schematically illustrates embodiments of a radial design ofthe photodiode according to the invention. The photodiode comprises anarray of nanowires 7 protruding from a semiconductor substrate 3. Ap-i-n junction is at least partly formed by a first region 1 made of asemiconductor material having a first conductivity type in thesemiconductor substrate and a second region 2 made of a semiconductormaterial having a second conductivity type opposite to the firstconductivity type surrounding each nanowire. A low doped or intrinsicregion 10 is provided in-between the array of nanowires 7 and saidsecond region 2 in order to provide alight absorption region forgeneration of charge carriers. It should be appreciated that the lightabsorption region of the active region also can be either p type or ntype as long as it is doped at a level low enough to be depleted atoperating bias. The first and second regions have opposite dopingpolarity, which in these embodiments is exemplified with an n-dopedfirst region and a p-type second region, however not limited to this.The formation of a radial intrinsic region is an alternative to theplanar intrinsic region and the radial approach simplifies the formationof a well defined intrinsic region. Moreover, the radial configurationwill enable a large depletion effect, forming a cylindrical depletionregion centered around the nanowire and extending out into thesurrounding shell layer.

In FIG. 2 a, the first region 1 comprises a first n-doped layer of inthe semiconductor substrate 3 and at least an end portion of each of thenanowires 7 is enclosed by a shell layer comprising the low doped region10 overgrown by a second p-doped layer at least partly forming saidsecond region 2. Preferably the nanowires 7 are made of a low doped orintrinsic semiconductor material. The second doped layer comprises asingle bandgap or a plurality of bandgaps, greater than the nanowirebandgap. A dielectric layer 15, or alternatively a low doped orintrinsic semiconductor layer, covers the surface of the semiconductorsubstrate 3 and surrounds the nanowires 7. The p- and n-doped layers andthe intermediate intrinsic region and intrinsic nanowire form a p-i-njunction. Charge carriers are photogenerated in a light absorptionregion 11 substantially corresponding to the low doped region 10 of theshell layer. As illustrate in FIG. 2 a the doped layer may be re-grownfrom the nanowires 7 to form a common coalesced semiconductor layerre-grown from nanowires and filling a gap between the nanowires 7. Thisfacilitates the formation of a common first contact 5 on the dopedlayer. This photodiode structure gives a high optical density incombination with a wide range of detectable wavelengths while stillretaining a low dark current due to the nanowire approach.

In FIG. 2 b a gap is left between the nanowires 7 when the second dopedlayer is re-grown from the nanowires 7. The gap may be filled with adielectric material, an intrinsic or a low doped semiconductor material.The material in the gap may be transparent. Due to the separation, eachnanowire 7 defines a separate photodiode joined to adjacent nanowirephotodiodes by the semiconductor substrate and optionally by a commonfirst contact arranged on the second doped layer.

In particular the photodiodes of FIGS. 2 a-b may comprise III-Vsemiconductor materials on Si, by way of example an n+-Si layer in asemiconductor substrate made of Si, intrinsic InGaAs nanowires, ap-InGaAs low doped region and a p+ InGaAs layer enclosing the low dopedregion. Such photodiodes can be exposed to light through the n+ Si layerif it is transparent to the incident photons or alternatively they canbe exposed to light from the top side through the p+ layer, if selectedwith a bandgap higher than the underlying layer. Since the lightabsorption region 11 of the photodiodes illustrated in FIG. 2 ispositioned at the nanowire side of the photodiode a multitude of otherIII-V materials can be used, and hence the bandwidth ranges from longwavelength IR (InSb, 0.17 eV) to high UV (AlN, 6.2 eV). The highflexibility of material choice is due to a fundamentally high toleranceof lattice mismatch that nanowires have been shown to exhibit.Furthermore, recent advances in epitaxial growth of III-V nanowiresdirectly on Si enables the fabrication of high quality III-V material onlarge, mass production-friendly, Si substrates, decreasing potentialfabrication-costs appreciably while avoiding the drawbacks of currentwafer bonding solutions.

The photodiodes of the invention may comprise a first contacting means 5that comprises a top contact electrically connected to the secondregion. When the photodiode is adapted to receive the light from thenanowire side a transparent top contact is preferred. Examples ofsuitable transparent contact materials are ITO and ZnO, however notlimited to this. Another example of a top contact is a heavily dopedsemiconductor, which has a wider bandgap and is transparent to incidentradiation, terminated with an ohmic contact.

In operation, the above described photodiodes generate a photocurrentdue to charge carrier generation in the light absorption region 11. Theactive region typically is within the low doped region, but may extendinto the surrounding p- and/or n-type regions and/or the nanowires. Thephoto generated charge carriers are inevitably directed to the nanowires7 and conducted away, due to the reverse bias polarity.

When charge carrier generation is in the substrate part of the p-i-njunction the maximum nanowire spacing should be increased to decreasethe dark current. However, the nanowire spacing is limited by increasedtrapping and recombination, and thus reduced collection efficiency, asthe spacing increases. Nanowires 7 protruding from a Si-substrate arepreferably spaced apart with a maximum spacing of less than 2 μm,preferably between 0.1 and 1 μm. In general, the maximum spacing may beexpressed in terms of the maximum diffusion length of the minoritycharge carriers. In one embodiment, when the light absorption region ispartially undepleted, the nanowires 7 are positioned with a maximumspacing between adjacent nanowires 7 that is less than the minoritycarrier diffusion length. The nanowires 7 may be arranged in an orderedarray with equal spacing between adjacent nanowires, but the nanowires 7may also be more randomly distributed although having an average spacingnot exceeding the preferred intervals.

The photodiodes according to the invention differ from a conventionalplanar device in that the one lateral layer is split into a large numberof columns. Advantage with this splitting is as mentioned above that thenanowire partitioning enables flexibility in selection of materials,bandgaps, doping profiles and/or levels and low defect density, whichenables improved leakage current, operating voltage and service life.

A fundamental advantage with nanowire photodiodes of the presentinvention is that the current path is partitioned without the loss ofactive volume, leading to a lower dark current without loss of quantumefficiency. The device physics is similar for the radial design and theaxial design, with the added benefit for the radial p-i-n structurebeing that the interface extends along the length of the nanowire andthat the crucial carrier separation takes place in the radial direction.The collection distance is expected to be smaller than the minoritycarrier diffusion length and thus the photo generated carriers can reachthe p-i-n junction with very high efficiency without substantial bulkrecombination. The partitioning of the photodiode may also decrease thecapacitance of the photodiode and thus increases the operation speed.

Both nanowires and nanostructures can be used in the photodiode to takeadvantage of the partitioning effect. In other words, the nanowire mayhave different lengths, i.e. it is a nanowie or a nanostub.

Referring to FIG. 3 a, the active region of the photodiode may besubstantially within the nanowires. In one embodiment of the presentinvention the photodiode comprises essentially intrinsic or low dopednanowires on a n-doped semiconductor substrate 3. An end portion of thenanowires 7, opposite to the substrate 3, are enclosed in a p-typesemiconductor material connected to a first contacting means 5.Consequently the depletion region is formed substantially in thenanowires. The depletion region may comprises the light absorptionregion and optionally also an avalanche multiplication region. Thebandgap of the nanowire may be designed by using heterostructures ofdifferent composition and/or doping level.

FIG. 4 schematically illustrates one embodiment of a nanowire-basedphotodiode comprising a p-i-n junction at least partly formed by a firstregion 1 made of a semiconductor material having a first conductivitytype and a second region 2 made of a semiconductor material having asecond conductivity type opposite to the first conductivity type, i.e.the first region is p-type and the second region is n-type or the firstregion is n-type and the second region is p-type. The p-i-n junctionfurther comprises a low doped region 10 between the first and secondregion 1,2 in order to form an light absorption region 11. The low dopedregion 10 is split into a first low doped layer and a second low dopedlayer by an array of nanowires 7 that are arranged to collect chargecarriers generated in a light absorption region 11. Said first region 1is formed by a first doped layer of the first conductivity type in asemiconductor substrate 3. The first low doped layer made of a low dopedor intrinsic semiconductor material of the first conductivity type isprovided between the nanowire array and said first region 1, typicallyas a surface layer arranged on the semiconductor substrate 3. Eachnanowire 7 protrudes through openings in a dielectric layer 15,alternatively a low doped or intrinsic semiconductor layer, to thesecond low doped layer of the second conductivity type. Said secondregion 2 is at least partly formed by a second doped layer of the secondconductivity type arranged on the second low doped layer. In operationcharge carriers generated from absorbed light in the second low dopedlayer are collected by the nanowires 7 and transported to the first lowdoped layer.

By way of example, in the photodiode structure of FIG. 4, the firstdoped layer comprises n+ Si, the first low doped layer comprises n− Si,the second low doped layer comprises p− InGaAs, and the second dopedlayer comprises p+ InGaAs.

FIG. 5 schematically illustrates one embodiment of a nanowire-based APDcomprising a p-i-n junction at least partly formed by a first region 1made of a semiconductor material having a first conductivity type and asecond region 2 made of a semiconductor material having a secondconductivity type opposite to the first conductivity type, i.e. thefirst region is p-type and the second region is n-type or the firstregion is n-type and the second region is p-type. The p-i-n or pnjunction further comprises an active region for generation of chargecarriers from absorbed light. The p-i-n junction further comprises a lowdoped region 10 between the first and second region 1,2 in order to forman active region. The low doped region 10 is split into a first lowdoped layer and a second low doped layer by one or more nanowires 7 thatare arranged to collect charge carriers generated in a light absorptionregion 11 of the active region. Said first region 1 is formed by a firstdoped layer of the first conductivity type in the semiconductorsubstrate 3. The first low doped layer made of a low doped or intrinsicsemiconductor material of the first conductivity type is providedbetween each nanowire 7 and said first region 1, typically as a surfacelayer arranged on the semiconductor substrate 3, in order to form anavalanche multiplication region 12. Each nanowire 7 protrudes throughopenings in a dielectric layer 15, alternatively a low doped orintrinsic semiconductor layer, to the second low doped layer of thesecond conductivity type, wherein the second low doped layer form alight absorption region 11. Said second region 2 is at least partlyformed by a second doped layer of the second conductivity type arrangedon the second low doped layer. In operation, charge carriers generatedfrom absorbed light in the second low doped layer are collected by thenanowires 7 due to a reverse bias and transported to the first low dopedlayer.

By way of example, in the APD structure of FIG. 5, the first doped layercomprises n+ Si, the first low doped layer comprises p− Si, the secondlow doped layer comprises p− InGaAs, and the second doped layercomprises p+ InGaAs. Such an APD utilizes the low noise multiplicationability of Si with high efficiency absorption InGaAs material. Inoperation, photons in the infrared region, to which the Silicon istransparent, typically incident from the substrate side, are absorbed inthe p-InGaAs layer. Under reverse bias the photogenerated electronsdrift towards the Si-part and are injected into the high-field p-Simultiplication region. A key advantage of this concept is that the highfield multiplication region is physically separated from the lower fieldlight absorption region. Without this separation, the dark current wouldbe dramatically higher due to Zener tunnelling in the InGaAs regionbecause of the small bandgap and low electron effective mass. A furtheradvantage of this design is that nanowires that are exposed to highfields are more prone to circuit breaking behavior rather than to shortcircuit.

Si nanowire APDs are interesting for communication devices due to thelow noise (avalanche noise) properties. In standard planar epitaxialgrowth technology it is not possible to form InGaAs—Si APDs because ofthe large lattice mismatch. Wafer bonding is currently the onlyalternative but results have been disappointing due to the large leakagecurrents.

FIG. 6 schematically illustrates in a) an electric field diagram for theembodiment of FIG. 5 under reverse bias and the band gap diagrams ofthis APD in b) without and in c) with bias. Note that the doping of thep-Si can also be either uniform or non uniform with a highly doped p+spike (not exceeding a few 100 nm in thickness) in an a low doped pregion. This so called Hi_LO APD configuration has the advantage ofachieving high gain at lower electric fields in the multiplicationregion because the field is nearly uniform in this region. The avalancheexcess noise is also smaller since the ratio of ionization coefficientsis higher at lower fields. If an intrinsic or low doped Si layer is usedinstead of a p− Si layer a doping spike should be introduced just awayfrom the interface to the p-InGaAs layer.

Such avalanche photodiodes can be designed so that:

-   -   the peak electric field is high enough to enable avalanche;    -   the photogeneration takes place in the p− InGaAs region, which        may be a radially coalesced layer enclosing the nanowires, and        electrons are injected into the p− Si region;    -   the p− InGaAs region is fully depleted; and    -   the electric field in the p− Si region at A in FIG. 6 is low        enough to prevent tunneling (Ei<1.5×10exp5V/cm for an InGaAs        composition having about 50% GaAs and 50% InAs).

The latter is critical since if the field is to high the dark currentrapidly increases with bias. Further tunneling is less likely with a lowgradient at C in FIG. 5

The photodiodes according to the invention is not limited to thepolarities shown. A pn configuration may be exchanged for a npconfiguration. A device with opposite polarities, with n-type wires maybe more convenient from a fabrication point of view. In the nanowire APDtwo or more materials can be used in the nanowire to reduceheterojunction barriers that will lead to carrier pile up effects thatcould slow down the device. For example a GaInAsP nanowire can be usedwherein the Ga and P content is varied along the nanowire. This isparticularly important for high speed communication (above ˜1 Gb/s).

As mentioned above, the invention makes it possible to providephotodiodes that have improved dark current properties making itpossible to make improved large-area devices. It is a further advantageof the invention to provide photodiodes that have a substantiallyconstant dark current over time, thereby significantly reducing the riskfor failure due to short circuiting. In particular the improved darkcurrent properties are useful for photodiodes operated at high reversebias levels, such as APDs, since deterioration of the semiconductormaterials is accelerated due to the formation of localized high electricfield spots, also known as microplasmas. This can lead to currentfilamentation, local heating and eventually premature device breakdown.Instead the nanowires of the photodiodes according to the invention maybe designed to provide a circuit breaker behavior. This failuremechanism manifests itself as a large increase of the nanowireresistance, usually by several orders of magnitude, followingapplication of a high electric field over the nanowire. The circuitbreaker behavior is advantageous over short circuiting behavior sincefailure of one or a smaller number of nanowires in a large array willnot significantly affect the performance while a short circuit may bedetrimental. Further, this behavior can be used to further improve theperformance and reliability of photodiodes according to the invention byutilizing at least a portion the nanowires as a current limitingstructure based on a space charge build-up in the nanowire portion. Thisspace charge effect yields a surprisingly high, and designable, seriesresistance which makes it possible to prevent excessive current leakageand device failure. Reverse resistance of the photodiode may also beincreased to enable photodiodes with higher breakdown voltage andimproved avalanche functionality.

The basic nanowire photodiode in accordance with the invention alreadyprovides improved current leakage and dark current properties. Theperformance, and in particular the current leakage and dark currentproperties, of the nanowire photo diode according to the invention canbe further improved by proper design of the photodiode in order to forman ideal spreading of the depletion region into the nanowire. Thecritical section of the photodiode is in the following referred to as acurrent limiting structure and the following description and estimationdemonstrates that several parameters, such as the dimensions of thenanowire, the doping levels, and semiconductor materials can be variedto obtain desired properties.

In one embodiment of the present invention a photodiode according to theinvention comprises a current limiting structure. The current limitingstructure comprises at least a portion of each nanowire 7 enclosed in adielectric material or a low doped or intrinsic material, whereby, inoperation, said portion is adapted to be at least partly depleted inorder to provide a predetermined space charge effect.

The current limiting effect can be designed by varying a number ofparameters such as: the spacing of the nanowires; the doping level; thediameter and the length of the nanowire portion; or by introducing aheterostructure in the nanowire or by changing materials combinations.

The current limiting structure can be designed without significantinfluence on the active region of the photodiode. Hence a large activeregion in combination with the current limiting structure can beobtained. In reverse bias mode, an active region of a photodiode isformed substantially in a low doped region adjacent to each nanowire inaccordance with the invention. The photodiode comprises a chargelimiting structure formed by a portion of the nanowire extending from aninterface to the low doped region. Due to the high electrical field thenanowire is at least partly depleted and a space charge is built up. Apredetermined current limiting effect can be obtained by control of theproperties, and the operation, of the p-i-n junction. Basically theeffect is dependent on to which extent the depletion region extends intothe nanowires. This can for example be controlled by the doping of thenanowires and the adjacent region. The current limiting structure may besmall, while having a large light absorption region.

A rough estimation of the space charge resistance in a nanowire gives:

$\begin{matrix}{{\nabla^{2}V} = \frac{J}{ɛ\; v_{D}}} & (1) \\{V = {{\frac{1}{2}\frac{J}{ɛ\; v_{D}}l^{2}} = {\frac{1}{2}\frac{I}{A}\frac{l^{2}}{ɛ\; v_{D}}}}} & (2) \\{R_{s} = {\frac{V}{I} = {{\frac{1}{2}\frac{l^{2}}{A\; ɛ\; v_{D}}} = {\frac{2}{\pi}{\left. \frac{\left( 10^{- 4} \right)^{2}}{\left( 10^{- 5} \right)^{2} \times 8.85 \times 10^{- 14} \times 10^{8}} \right.\sim 10}\mspace{14mu} M\; \Omega}}}} & (3)\end{matrix}$

V is the voltage across the wire length l, assumed to be about 1 micron;J and I are the nanowire current density and current. A is the nanowirecross-sectional area (assumed to correspond to a diameter of about 100nm), v_(D) is the carrier drift velocity taken to be about 10⁷ cm/s. Thepermittivity of the semiconductor NW is assumed to be about 10.

The estimation shows that the space charge resistance is very high andthis can be used to:

-   -   minimize internal leakage currents, i.e. dark current;    -   increase the resistance to levels not readily possible with        planar technology:    -   establish huge space charge effect in forward biased p-i-n        devices;    -   prevent local fatal device failure, since space charge does not        affect reverse biased devices unless there is surge (rapid        current rise near the breakdown voltage);    -   design I-V-characteristics; etc.

Different implementations of a nanowire photodiode according to theinvention can be envisaged in:

-   -   communication;    -   large area, low ID, devices on Si; in particular long wavelength        devices    -   mid-IR devices, in particular cameras; and    -   radiation hardened devices, in particular using nanowires        comprising nitride semiconductors such as GaN.

Referring to FIG. 7, a well known problem in planar technology is that adoping profile is created that yields locally higher electric fields atthe device perimeter, resulting in premature breakdown. To avoid this, aguard ring is used in planar avalanche photodiodes. The nanowireapproach eliminates the need for the guard ring since a hemisphericaldoping profile in the substrate in connection with each nanowire yieldsa homogenous electric field at the same distance from the nanowire. FIG.7 schematically illustrates an array of nanowires protruding from asubstrate and having a hemispherical doping profile under each nanowire.This hemispherical doping profile can be realized in many ways, forexample by dopant diffusion during nanowire growth, or dopantimplantation or diffusion previous to nanowire growth. An alternativeconfiguration is to have a doping profile extending between the bases ofthe nanowires, as to give a homogeneous planar doping under thenanowires to avoid the guard ring. This adds one fabrication step,before growth or in situ before nanowire growth. Both nanowires andnanostubs can be used to eliminate the guard-ring but in order to obtaincircuit breaker behavior, nanowires should be used.

The present invention enables heterojunction PDs wherein practically“any” semiconductor materials can be combined. This is possible becausenanowires can be grown on semiconductor substrates or layers regardlessof the lattice mismatch. Materials that can be used can range from, atleast InSb (0.17 eV) to AlN (6.2 eV), allowing photocurrent responsefrom the far infra-red into the ultra-violet. In general the material ofthe absorbing regions is the same of that of the nanowire or nanostubalthough if the absorbing material has small bandgap it will be helpfulto have the nanowire of higher bandgap to further reduce the darkcurrent with a graded interface with the absorbing layer to allow goodcarrier collection through the nanowires.

Although the embodiments have been described with InGaAs/Si structuresas an example, other III-V materials can be used, such as for exampleInAsP and any different combinations of In, Ga, As, and P, and othersemiconductor substrates may be used. As appreciated by a person skilledin the art, the stochiometric composition of the compounds can bevaried. Suitable materials for doping are well known in the art.

A semiconductor material without significant dopant species present iscommonly referred to as an intrinsic semiconductor material, which isdesignated as i in the p-i-n junction. For the purpose of thisapplication it is to be appreciated that an intrinsic region or materialalso can be a region with carrier concentration substantially comparableto or equal to the intrinsic value, unintentionally doped or low dopedp-type or n-type.

The different regions in a photodetector may consist of several layersof different composition or even layers with compositional grading orhaving a graded doping profile. While, the embodiments in thisapplication are described without such heterojunctions it is intended tobe within the scope of the application. By way of example only, a pn orp-i-n junction may be obtained by having the core-shell structuredescribed above and using a heterostructure segment in the nanowiresinstead of a doped semiconductor substrate. Furthermore, the devices cangenerally be designed to work in avalanche mode although this is notexplicitly disclosed in the description.

Surface layer is used throughout this application to denote asemiconductor layer arranged on a semiconductor substrate. Such layersare commonly referred to as buffer layers, which often provideone-dimensional properties due to limited thickness of the layer.Surface layers according to the invention are neither limited inthickness nor to one-dimensional properties. On the contrary, thesurface layer may have bulk-like properties.

Although the present invention have been described in terms of “top”,“vertical”, “back”, and the like, the physical orientation in space ofthe photodetector device is not of importance. These terms are only usedto describe the interrelations between different features of thephotodetector.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments, on the contrary, it is intended to cover variousmodifications and equivalent arrangements within the appended claims.

1. A photodiode comprising a p-i-n or pn junction at least partly formedby a first region comprising a semiconductor material having a firstconductivity type and a second region comprising a semiconductormaterial having a second conductivity type opposite to the firstconductivity type, wherein the p-i-n or pn junction comprises a lightabsorption region for generation of charge carriers from absorbed light,and wherein one section of the p-i-n or pn junction comprises an arrayof nanowires that are spaced apart and arranged to transfer said chargecarriers from the light absorption region.
 2. The photodiode accordingto claim 1, wherein: the p-i-n or pn junction comprises at least a firstlow doped region comprising a low doped or intrinsic semiconductormaterial provided between the nanowires and one of said first region andsaid second region, and the light absorption region is substantiallywithin said first low doped region.
 3. The photodiode according to claim2, wherein the p-i-n or pn junction further comprises an avalanchemultiplication region for multiplication of charge carriers generated inthe light absorption region.
 4. The photodiode according to claim 2,wherein the avalanche multiplication region and light absorption regionsubstantially are within the first low doped region.
 5. The photodiodeaccording to claim 1, comprising: a first low doped region made of a lowdoped or intrinsic semiconductor material provided between the nanowiresand said first region and a second low doped region made of a low dopedor intrinsic semiconductor material provided between the nanowires andsaid second region, wherein the second low doped region is arranged tocontribute to the formation of a light absorption region for generationof charge carriers and the first low doped region is arranged tocontribute to the formation of an avalanche multiplication region. 6.The photodiode according to claim 1, wherein: the nanowires protrudefrom a semiconductor substrate or a surface layer arranged on thesemiconductor substrate; and the semiconductor substrate or the surfacelayer comprises said first region.
 7. The photodiode according to claim6, wherein: said first region of first conductivity type is a dopedregion in the semiconductor substrate or the surface layer; and thefirst low doped region is provided between said first region and thenanowires.
 8. The photodiode according to claim 3, wherein: said secondregion of second conductivity type is at least partly formed by thearray of nanowires; said first region is at least partly formed by afirst doped layer of the first conductivity type in the semiconductorsubstrate; and a first low doped layer made of a low doped or intrinsicsemiconductor material of the first conductivity type is providedbetween the array of nanowires and said first region in order to atleast partly form said first low doped region.
 9. The photodiodeaccording to claim 2, wherein: said first region is formed by a firstdoped layer of the first conductivity type in the semiconductorsubstrate; a first low doped layer made of a low doped or intrinsicsemiconductor material is provided between the nanowires and said firstregion; the nanowires protrude through openings in a dielectric layer ora low doped or intrinsic semiconductor layer to a second low doped layerof the second conductivity type, wherein the second low doped layer formthe light absorption region; and said second region is at least partlyformed by a second doped layer of the second conductivity type arrangedon the second low doped layer; whereby charge carriers generated fromabsorbed light in the second low doped layer are collected by thenanowires and transported to the first low doped layer.
 10. Thephotodiode according to claim 9, wherein: the first doped layercomprises n+ Si; the first low doped layer comprises n− Si; the secondlow doped layer comprises p− InGaAs; and the second doped layercomprises p+ InGaAs.
 11. The photodiode according to claim 9, wherein:the first doped layer comprises n+ Si; the first low doped layercomprises p− Si; the second low doped layer comprises p− InGaAs, and thesecond doped layer comprises p+ InGaAs; and the first doped layer isadapted to form the avalanche multiplication region.
 12. The photodiodeaccording to claim 1, wherein at least a portion of each nanowire isenclosed by a shell layer comprising the low doped region overgrown by asecond doped layer, the second doped layer at least partly forming saidsecond region of the p-i-n junction.
 13. The photodiode according toclaim 1, further comprising a current limiting structure, wherein thecurrent limiting structure comprises at least a portion of each nanowireenclosed in a dielectric material, an insulating material or a low dopedor intrinsic material, whereby, in operation, said portion is adapted tobe at least partly depleted in order to provide a predetermined spacecharge effect.
 14. The photodiode according to claim 13, wherein thenanowire portion of the current limiting structure is doped.
 15. Thephotodiode according to claim 1, wherein the nanowire comprises a lowdoped or intrinsic semiconductor material.
 16. The photodiode accordingto claim 1, wherein the nanowire comprises III-V semiconductor material.17. The photodiode according to claim 1, wherein the light absorptionregion comprises one or more of the semiconductor materials selectedfrom the group of InGaAs, InAsP and InGaAsP.
 18. The photodiodeaccording to claim 6, wherein the semiconductor substrate comprises Si.19. The photodiode according to claim 1, further comprising atransparent contact electrically connected to one of said first regionand said second region.
 20. The photodiode according to claim 1, whereinthe nanowires are arranged to provide an avalanche multiplicationregion.
 21. The photodiode according to claim 6, wherein thesemiconductor substrate or the surface layer comprises a hemisphericaldoping profile in connection with each nanowire.